Solenoid valves are a known type of electronically activated valves. Solenoid valves are often used to control the flow of liquids or gases, replacing manual valves where a remote control of the valve is desirable. A solenoid valve is operated by providing a coil with electrical current, whereby a valve member is displaced inside the housing to open or close an orifice, whereby a flow through the valve is permitted or prevented. The orifice is commonly opened or closed by displacing a valve element from a valve seat or displacing the valve element onto the valve seat.
In case of a normally closed valve, the valve element is usually maintained against the valve seat by a return spring, preventing flow through the solenoid valve as long as electric current is not provided to the solenoid valve.
For directly operated solenoid valves, the main orifice is the only flow path through the valve. Such directly operated solenoid valves are used in systems where either low flow capacities are sufficient or if the maximum operating pressure differential between the inlet and the outlet pressures of the valve are comparatively low. In case the pressure differential between the inlet and the outlet becomes too large, directly operated solenoid valves cannot be operated reliably anymore. In particular, the pressure acting on the valve member or the valve element may become too large to allow a controlled opening or closing of the solenoid valve, resulting in the valve element either not being displaceable or displacement of the valve element requiring much force and possibly resulting in damage to the valve.
Problems mentioned above are less severe with piloted solenoid valves, wherein the solenoid valve allows or prevents fluid connections between different chambers of the valve. Thereby, the main flow connection between inlet and outlet is only opened, if a minimum operating pressure differential between the inlet and the outlet is present, even if the solenoid valve is open.
A solenoid valve as mentioned above is known from U.S. Pat. No. 7,828,265 B2. The solenoid valve is normally closed and the valve element may be displaced away from the valve seat by providing a coil with electric current. The valve element is received within a bore of a slideable valve member. The valve element is forced towards the valve seat by two coaxial springs. The coaxial springs are received in different sections of the bore of the valve member to ensure a controlled displacement of the valve member in relation to the valve element. If the coil is provided with electrical current, the valve member is displaced away from the valve seat and engages the valve element to displace the valve element from the valve seat in a controlled manner. A forceful impact of the valve member on the valve element is prevented by the use of the two coaxial springs. The solenoid valve may be used at a higher maximum operating pressure differential than previous solenoid valves.
The solenoid valve shown in U.S. Pat. No. 7,828,265 B2 has the disadvantage of a complicated construction of the solenoid valve, in particular two different types of springs must be positioned coaxially in bores of the valve member having different diameters. An adjustment of the spring properties requires changing the length and/or diameter of the bores.
An additional problem of the solenoid valve shown in U.S. Pat. No. 7,828,265 is that if an electrical current to the coil is not provided, some residual magnetization of the valve member may remain, which may counteract the spring forces trying to force the valve member towards the closing position. Same problem may be caused by adhesion forces between the valve member and the anchor core, against which the valve member abuts in the open position of the valve. Also the adhesion forces must be overcome for displacing the valve member towards the closing position. Both problems may become more severe with the number of duty cycles that the valve has performed.
A further solenoid operated proportional valve is known from U.S. Pat. No. 3,926,405. This valve has a valve element pressed against a valve seat by means of a coil spring. The valve element can be lifted off from the valve seat by means of an auxiliary element which comes into contact with a progressive spring after a certain distance of movement in opening direction.
US 2012/0223261 A1 shows a switching valve having a valve element moveable in a housing. The valve element is loaded in closing direction by a series connection of two springs. The two springs are separated by means of an adapter piece. After a first part of a stroke of the valve element in opening direction the adapter piece comes to contact with a stop in the housing of the valve. The following movement of the valve element takes place against the force of a single spring only. Assembly of such a valve is rather complicated.
U.S. Pat. No. 6,076,550 shows a solenoid and a solenoid valve which is pilot controlled. A valve element is connected to a plunger, said plunger being moveable against the force of a spring within a coil of a solenoid. The attracting force in opening direction is increased by means of a permanent magnet.
DE 199 48 446 A1 shows a progressive spring for a suction valve, in particular for pumps of a controlled breaking system.